Immersion lithography is a resolution enhancement technique for exposing images on a substrate such as a surface of a semiconductor wafer. Immersion lithography interposes a high refractive index fluid, such as water, between a projection lens of a lithography system and the substrate. The substrate is typically positioned on a stage or holder, and the fluid is maintained between the projection lens and the stage by a fluid containment mechanism, e.g., a set of “shower heads.” An image can then be projected, such as with deep ultra-violet (DUV) radiation, through the projection lens, through the immersion fluid, and then be exposed onto the substrate surface.
In order to isolate the fluid from the substrate surface during loading and unloading, a fluid closing plate may be used. During exposure, the fluid closing plate is placed in a plate holder of the stage. Prior to wafer unloading, the fluid closing plate is lifted from the plate holder and affixed to the fluid containment mechanism. In this way, the fluid is isolated from the substrate surface while the substrate is unloading. After a new substrate is loaded onto the stage, the fluid closing plate is resituated in the plate holder. This allows the immersion fluid to re-interpose between the projection lens and the substrate surface. The process in which the fluid closing plate is dislocated from the plate holder and resituated is referred to as “closing plate take-over.”
Currently, the position of the fluid closing plate is calibrated after a defined number of cycles using a transmission image sensor (TIS). The transmission image sensor is mounted inside the substrate holder at a distance from the plate holder. The transmission image sensor calibrates the plate position by detecting the DUV radiation projected from the projection lens and adjusting the plate position based on the distance between the detected radiation and the plate holder.
However, this type of calibration is not efficient because extra maintenance time is needed to adjust the plate position if it is out of specification. In addition, the calibration does not monitor plate position after every closing plate take over. This may result in a collision of the fluid closing plate with the wafer holder or other materials, which may lead to damage of parts, unwanted particles, and machine downtime.
A need exists for a method and system that monitor the position of the fluid closing plate after every closing plate take-over. In this way, collision with the wafer holder or other materials may be avoided and efficiency of immersion lithography may be improved.